The present invention relates to a speech control circuit, and, in particular, relates to such a circuit which is utilized in a loudspeaker telephone or a handsfree telephone having a microphone and a speaker, and provides the correct switching of the transmission mode and the reception mode in spite of the acoustic coupling between the speaker and the microphone.
A loudspeaker telephone has a transmission circuit which transmits a signal from a microphone to a circuit line, and a reception circuit which receives the signal from the circuit line and energizes a speaker. Both the transmission circuit and the reception circuit have variable insertion loss circuits, or varialossers which are controlled by a speech control circuit, so that when a man speaks to a microphone, the insertion loss in the reception circuit is large and the insertion loss in the transmission circuit is small, and when the reception signal from the circuit line exists, the insertion loss in the reception circuit is small and the insertion loss in the transmission circuit is large. Thus, the transmission mode and the reception mode are substantially switched by a speech control circuit according to the transmission signal and the reception signal.
In a speech control circuit, three operational modes are defined. "Static mode" is the mode that no transmission signal nor reception signal exists. "Reception mode" is the mode that the reception signal from the circuit line exists, but no transmission signal exists. "Transmission mode" is the mode that the transmission signal from a microphone exists, but no reception signal from the circuit line exists.
In a static mode, when a reception signal from the circuit line is provided, that signal is acoustically generated from a speaker, and then, the acoustic signal from the speaker is applied to a microphone through the air space between the speaker and the microphone. Therefore, the speech control circuit will detect the transmission signal, even though only the reception signal is applied. In this case the speech control circuit must be composed so that the level of the reception signal is higher than the level of the transmission signal. Otherwise, the system would be switched to the transmission mode by the reception signal. That situation is called "error by acoustic coupling". In particular, when both a microphone and a speaker are mounted in a single housing, the acoustic coupling between a microphone and a speaker is rather large, so that error by acoustic coupling has been a serious problem.
A prior speech control circuit for preventing the acoustic coupling is shown in FIG. 1.
In the figure, the transmission speech signal generated in a microphone M is amplified by a microphone amplifier MA, the output of which is applied to a hybrid circuit MYB which converts a four wire signal to a two wire signal and vice versa, through a transmit variolosser TVL, which is a variable attenuator. Also, the reception signal from the transmission line is applied to a speaker SP through the hybrid circuit HYB, a receive variolosser RVL and a speaker amplifier SA. The acoustic signal generated by the speaker is dispersed in the air. The hybrid circuit functions to transmit the output of the transmit variolosser to the line, and the signal on the line to the input of the receive variolosser. In an ideal condition, the output of the transmit variolosser does not leak to the input of the receive variolosser. However, in an actual hybrid circuit, some small ratio .beta. of the output of the transmit variolosser leaks to the input of the receive variolosser. That leak in the hybrid circuit is an important factor in analyzing the operation of the speech control circuit.
The speech control operation in the above circuit will be described below.
The speech signal from the microphone M is obtained at the output of the microphone amplifier MA. The output of the microphone amplifier MA is further amplified by the transmit control amplifier AT, the output voltage V.sub.t ' of which is applied to the comparator COMP. Similarly, the received speech signal from the line is picked up at the output (a) of the hybrid circuit HYB and the output (b) of the speaker amplifier SA, and those outputs are amplified by the first reception control amplifier AR.sub.1 and the second reception control amplifier AR.sub.2, respectively. The outputs of those reception control amplifiers AR.sub.1 and AR.sub.2 are combined or added to each other by the adder R, and the output voltage V.sub.r ' of said adder R is applied to the comparator COMP.
The comparator COMP compares the voltage V.sub.t ' with the voltage V.sub.r ', and switches the operational mode between the transmission mode and the reception mode through the control of the insertion loss in the transmit variolosser TVL and the receive variolosser RVL according to the result of the comparison. That is to say, when V.sub.t '.gtoreq.V.sub.r ' is satisfied, the mode is determined as the transmission mode, and the insertion loss of the transmit variolosser TVL is controlled so as to be small or preferably zero, and the insertion loss of the receive variolosser RVL is controlled so as to be large. On the other hand, when V.sub.t '&lt;V.sub.r ' is satisfied, the mode is determined as the reception mode, and the insertion loss of the transmit variolosser TVL is controlled so as to be large, and the insertion loss of the receive variolosser RVL is controlled so as to be small.
The comparator COMP controls the operational mode to the reception mode when both V.sub.t ' and V.sub.r ' are lower than the predetermined value.
The gain of the first reception amplifier AR.sub.1 is relatively small, and the gain of the second reception amplifier AR.sub.2 is relatively large. The reason for that is described below.
When the speech signal is received from the line, the speech signal is converted to an acoustic signal by the speaker SP. The acoustic signal is then applied to the microphone M through the air space with the loss .alpha.. The signal thus applied to the microphone M is re-converted to an electric signal, which is applied to the microphone amplifier MA and the transmission amplifier AT. And the output voltage V.sub.t ' of the transmission amplifier AT is applied to the comparator COMP. Of course, the reception signal is also amplified by the first reception amplifier AR.sub.1 and the second reception amplifier AR.sub.2, and the adder provides the combined output voltage V.sub.r. In this case, in order to keep the reception mode, the relationship V.sub.r '&gt;V.sub.t ' must be satisified. In order to satisfy the above relationship, the gain of the second reception control amplifier AR.sub.2 must be larger than the difference between the sum of the sensitivity of the microphone M, the gain of the microphone amplifier MA and the gain of the transmission control amplifier AT; and the acoustic loss .alpha.. Therefore, the gain of the second reception control amplifier AR.sub.2 must be relatively large.
In this case, if the gain of the first reception control amplifier AR.sub.1 is large, instead of the second reception control amplifier AR.sub.2, the error by the side tone .beta. in the hybrid circuit HYB occurs. That is to say, the output of the transmit variolosser TVL leaks to the input of the reception variolosser RVL through the side tone .beta. of the hybrid circuit HYB, because the hybrid circuit HYB is not an ideal one. Said side tone would be amplified by the first reception control amplifier AR.sub.1, which would provide the high level of V.sub.r ', thus, the mode would be switched to the reception mode although the correct mode is the transmission mode. Therefore, the gain of the first reception control amplifier AR.sub.1 must be low so that the system does not operate incorrectly due to the side tone .beta. of the hybrid circuit in the transmission mode. Also, the gain of the first reception control amplifier AR.sub.1 must be enough to switch the mode from the transmission mode to the reception mode when the reception signal appears at the input (a) of the reception variolosser RVL during the transmission mode. Since, the level of the reception signal is usually high, the gain of the first reception control amplifier AR.sub.1 can be low. Due to the presence of the first reception control amplifier AR.sub.1, the mode can be changed quickly and the omission of the beginning of the received speech is prevented when the mode changes from the transmission mode to the reception mode.
The above operations are summarized as follows.
(a) In a static mode, both the levels V.sub.t ' and V.sub.r ' are low, and the comparator COMP determines the mode to be the reception mode.
(b) In a transmission mode, the relationship V.sub.t '.gtoreq.V.sub.r ', is satisfied; thus, the attenuation by the transmit variolosser TVL is low and the attenuation by the receive variolosser RVL is high.
(c) In a reception mode, the relationship V.sub.t '&lt;V.sub.r ' is satisfied, and thus, the attenuation of the transmit variolosser TVL is high, and the attenuation of the receive variolosser RVL is low. In this case, in order to prevent error through acoustic coupling, the gain of the second reception control amplifier AR.sub.2 must be sufficiently high.
(d) When a reception signal appears during the transmission mode, that reception signal is amplified by the first reception control amplifier AR.sub.1, which provides the level of V.sub.r ' higher than V.sub.t ', and thus, the operational mode is quickly switched from the transmission mode to the reception mode.
However, the prior speech control circuit shown in FIG. 1 has the following two disadvantages.
(1) The addition by the adder R is carried out for the alternate waveforms. However, since the phase of the output of the first reception control amplifier AR.sub.1 does not completely coincide with that of the second reception control amplifier AR.sub.2, the sum (V.sub.r) of the adder R is not high enough. FIG. 2(a) shows the case where the phase of the first reception control amplifier AR.sub.1 coincides completely with that of the second reception control amplifier AR.sub.2, and the sum V.sub.r ' at the output of the adder R is sufficiently high. On the other hand, FIG. 2(b) shows a case where there exists the phase difference or the time delay .tau. between the outputs of the first reception control amplifier AR.sub.1 and the second reception control amplifier AR.sub.2. In this case, it is apparent that the level of the output of the adder R is considerably lower than that of the case of FIG. 2(a). That time delay .tau. is caused by the propagation time in the reception variolosser RVL and the speaker amplifier SA. The resultant sum V.sub.r ' at the output of the adder R is sometimes reduced almost to zero depending upon the operational frequency. The reduction of that level V.sub.r ' causes the error in the operation of the switching.
(2) The system sometimes operates incorrectly due to the noise from the circuit line. That is to say, since the gain of the second reception control amplifier AR.sub.2 is very high as mentioned above, noise from the circuit line is also amplified by said second reception control amplifier AR.sub.2 enough to provide the relationship V.sub.r '.gtoreq.V.sub.t '. If that situation occurs, the system is switched incorrectly by the noise. That incorrect operation is called a noise block.